Modern HDTV broadcasts will be required to co-exist with the broadcast of NTSC (National Television Systems Committee) television broadcast signals presently in use. Since simulcasting will be used to accommodate HDTV broadcasts, an HDTV receiver will receive both an intended HDTV broadcast along with an unwanted NTSC broadcast. A problem of interference results from the presence of the NTSC broadcast being received, along with the HDTV broadcast signal.
NTSC interference caused by neighboring NTSC transmitters will be most severe at the fringe coverage area of the HDTV transmission region. At the fringe coverage area, the signal to noise ratio (SNR) of the HDTV broadcast will be at its lowest, with respect to the HDTV coverage area. Such a low SNR of the HDTV broadcast results in this fringe area since this is the broadcast region that is the greatest distance from the HDTV transmitter and, thus, it is an area which receives a relatively weak HDTV signal. Also, in the fringe coverage area the interfering NTSC signal will be at its largest, with respect to the coverage area of HDTV, since the distance to the NTSC transmitter increases and NTSC signal strength decreases, the further the NTSC broadcast signal travels away from the fringe area into the HDTV coverage area.
Thus, it is particularly in this fringe coverage area where NTSC broadcasts from neighboring broadcast regions will cause the most significant interference with an HDTV transmission.
Several proposed HDTV receivers use what is referred to as a baseband demodulator architecture. Such systems include a tuner for receiving a television signal which may include a HDTV signal and NTSC signal interference. An I.F. section is used to generate an I.F. signal from the television signal received by the tuner. In such systems, the generated I.F. signal may include the NTSC signal interference along with the desired HDTV signal. The proposed HDTV systems mix the I.F. signal down to baseband using analog techniques and a complex signal consisting of cos (.omega.t) and sin (.omega.t) waveforms to generate in-phase (hereinafter "I-") and quadrature phase (hereinafter "Q-") signals. To remove second order products, identical lowpass filtering is then performed on the resulting I- and Q- signals. The combination of the in-phase and quadrature components constitute a one sided analytic signal. In the proposed systems, this complex waveform consisting of the I- and Q- signals is then converted into the digital domain by two simultaneous analog to digital ("A/D") converters. Once in the digital domain, traditional demodulation processing is performed on the resulting signals, e.g., adaptive equalization, bit carrier synchronization, etc.
Approaches to solve the problem of NTSC signal interference should effectively eliminate the NTSC interference without severely attenuating what may be an already weak HDTV signal.
One known approach to solving the NTSC signal interference problem is the use of a comb filter which has notches spaced apart by a fixed amount each notch having a fixed depth and width to eliminate NTSC interference. In accordance with this approach, the received signal is first demodulated down to the baseband signal and then filtered. Such an approach has several disadvantages. For instanced the use of a comb filter degrades the noise performance of the system by 3 decibels (dB). This loss in detection probability is due to the very wide notches that are placed at the location of the NTSC interference carrier and is also due, in part, to the fact that the comb filter also contains additional notches where no interference is located. These additional notches cause the unnecessary attenuation of the HDTV signal in those areas where the additional notches are located, resulting in a reduced probability of signal detection.
Another known approach to resolving the problem of NTSC signal interference is to use an adaptive equalizer which forms notches which are then used to eliminate NTSC interference from the HDTV signal. Such a system can be both difficult and costly to implement.
Another known approach to the problem of NTSC signal interference uses spectral shaping of the HDTV signal to avoid all but the interference caused by the NTSC chrominance subcarrier. This approach fails to remove all of the NTSC signal interference from the HDTV signal and permits some of the chrominance subcarrier signal to remain causing interference with the HDTV signal. A further disadvantage of this approach is that it fails to make the most efficient usage of the available broadcast spectrum space.
In addition to the above approaches, forward error correction (FEC) encoding has also been used to combat NTSC signal interference. This approach, which attempts to correct errors resulting from NTSC signal interference, rather than to remove such interference from the received signal, has the drawback that it may not be possible to correct all the errors caused by the interference. Furthermore, the use of large amounts of FEC encoding fails, as with the case of spectral shaping to avoid NTSC signal interference, to make the most effective use of the available broadcast spectrum space.
The known approaches to NTSC signal cancellation can be difficult and costly to implement. Furthermore, several of the known systems fail to permit efficient use of the available broadcast spectrum space.